Process for continuously preparing a polyamide 6 with a low dimer content

ABSTRACT

The invention concerns a process for continuously preparing polyamide 6 with a low dimer content from ε-caprolactam (fresh lactam) and/or from recycled lactams from extraction and melt-demonomerizing stages. In a plurality of process stages the recycled lactam and/or fresh lactam is hydrolyzed under pressure in the fusible state with water contents of between 3 and 15% and temperatures ranging from 220° C. to 280° C. With a lower constant water content being set, the molten polyamide is then fed to the polymerization process in a further pressure stage incorporating a gas chamber.

BACKGROUND OF THE INVENTION

The invention relates to a multistage process for continuously preparinga polyamide 6 with a low dimer content from ε-caprolactam and/or fromrecycled lactams (so-called return lactams) which are obtained inextraction stages and melt demonomerization stages for reducing the lowmolecular mass fraction in the case of polyamide 6.

It is known that during the polycondensation of ε-caprolactam to givepolyamide 6 a chemical equilibrium is established which istemperature-dependent. At a common plant reaction temperature of 280°C., a water-soluble fraction of around 13% is established.

For the further processing of the polymer it is necessary to dissolveout the unreacted fraction. Various methods are known for this purpose.For example, the melt in the equilibrium state is used to preparegranules which are extracted with water. In that case ε-caprolactam,oligomers and dimers are dissolved out. This method necessitatessubsequent drying of the granules and processing of the aqueousextraction waters by evaporative concentration.

Also known are methods in which unreacted ε-caprolactam, dimers andoligomers are driven off from melt films by means of reduced pressure orinert gas.

For economic reasons, these recovered products should be supplied to thepolymerization process again.

There are methods in which PA 6 extract water concentrates with high orlow solids contents are added in smaller or greater proportions to thefresh lactam in order that they too may be reacted to give polyamide 6.In accordance with DE-B 25 01 348 and DE-A 27 32 328 polyamide 6 extractwaters concentrated to a solids content of more than 90% or,respectively, to 60-70% are polymerized with more than the equal amountof fresh lactam, based on the solids content, with propionic or,respectively, benzoic acid in a reactor which is operated essentially atatmospheric pressure.

A disadvantage of this process is that when polyamide 6 extract waterconcentrates are polymerized with monocarboxylic acids, thepolymerization times required are longer than in the case of polymers offresh lactam with monocarboxylic acids. In addition, the conversions ofε-caprolactam or cyclic oligomers obtained are from about 2 to 3% belowthose of fresh lactam polymers, since the cyclic oligomers takeessentially no part in the reaction mechanism. The high water content inthe reaction mixture, in conjunction with the pressureless operation ofthe polymerization reactor, leads to an unfavorable reaction course. Thewater is unable to act in the reaction because it evaporates.

It is also known that melt vacuum demonomerization processes areoperated such that the demonomerization products drawn off in gas form,i.e. ε-caprolactam and cyclic oligomers, are condensed in fresh lactamand so also subjected to the polymerization (DE-A 28 21 886 and DE-A 2948 865). In the case of similar processes as well, the cyclic oligomers,especially the cyclic dimers, are not converted into reactive compounds.

Also known is the strong catalytic action of o-phosphoric acid on thepolymerization of ε-caprolactam [Geleji, F. et al. "Wirkungsmechanismusverschiedener Katalysatoren bei der Caprolactampolymerisation"[Mechanism of action of various catalysts in the polymerization ofcaprolactam] in Faserforschung und Textiltechnik 13 (1962) 6, 282-283]in terms of the polymerization of cyclic oligomers (Schade, H. andBeckhaus, F. W.: Verfahren zum Polymerisieren von Oligomeren desε-Caprolactams [Process for polymerizing oligomers of ε-caprolactam]--DDPatent 24 307). Disadvantages of such a polymerization technology thatoccur are, inter alia:

excessively rapid increase in viscosity and hence inadequatereproducibility of the desired degree of polymerization, since theo-phosphoric acid acts essentially as catalyst and to a lesser extent aschain regulator

inadequate conversion to ε-caprolactam and cyclic oligomers, and

depolymerization processes when polyamide 6 granules of this kind areprocessed.

In order to avoid the abovementioned problems it is common to separateoff the oligomers when processing the aqueous extraction waters bydistillation. The oligomers separated off are landfilled, which pollutesthe environment, or undergo energy-intensive depolymerization intolactam, by processing with phosphoric acid, and the lactam issubsequently passed to a cleaning stage. The residues are land-filled.

For technical reasons concerned with the apparatus and on economicgrounds, polyamide manufacturers have to date concentrated the lactamoligomer mixture to a residual water content of ≦2% by evaporativeconcentration. With this water content it becomes impossible to bind thecyclic dimer chemically into the polymer structure.

The aim of the invention is to restrict as much as possible theformation of dimers and oligomers in the preparation of polyamide 6 fromfresh lactam and to bind cyclic dimers, especially cyclic dimersintroduced by way of recycled lactam, and also cyclic oligomers into theproduction process and to obtain the processing of the total amount togive high-grade end products while creating a closed circuit ofmaterials (no waste).

SUMMARY OF THE INVENTION

The object on which the invention is based is to hinder the formation ofthe cyclic dimer in the course of the polymerization and, at the sametime, to convert existing cyclic dimers of the lactam-oligomer mixturefrom recycled lactam into a reactive compound, in a simple manner, andto allow them to participate in the preparation process.

In accordance with the invention, fresh lactam and/or recycled lactam isintroduced into a preferably closed system and is treated by hydrolysisin the liquid melt phase under pressure at water contents of from 3% to15% and at temperatures in the range from 220 to 280° C. A treatment ofthis kind applied to fresh lactam and/or recycled lactam in a pressurestage is preferably undertaken before these components pass throughreaction stages of a polymerization plant.

In the case of the preparation of polyamide 6 from fresh lactam, in afirst pressure stage the hydrolysis of the ε-caprolactam is undertakenin a closed system having preferably only an entry pipe and an exitpipe, in the liquid melt phase at water contents in the range from 3-10%and at temperatures between 220 and 280° C.

In the case of the preparation of polyamide 6 from fresh lactam orrecycled lactam alone, the oligomers are dissolved in the lactam in thereaction mixture, preferably even before entering the first pressurestage. In this case, depending on the initial concentration of theoligomers in the solution, a solution temperature of the oligomers inthe lactam of between 120 and 180° C. is established.

In the first pressure stage (dimer treatment stage) the hydrolysis offresh lactam and recycled lactam, or recycled lactam alone, isundertaken in the liquid melt phase at a temperature in the range from220 to 280° C. In the course of the hydrolysis of the cyclic oligomers,especially cyclic dimers, in the reaction mixture in this pressurestage, the water content is held, corresponding to the water content ofthe recycled lactam, at from 6 to 15%.

With the above establishment of water concentration and reactiontemperature, the dimeric ring compound is cleaved open. In this caseaminocaproic acid and linear dimers are obtained from the cyclic dimer,which allow trouble-free insertion into a polymer chain.

It has surprisingly been found that about 3% of water must be present inthe lactam-oligomer mixture in order to cleave the cyclic dimer, withthe water component making a favorable and fully effective contributionto the reaction under increased pressure in a closed system. With such asolution it is possible to employ fresh lactam and recycled lactam andalso, without problems, even recycled lactam alone.

The water content in the polyamide 6 melt is subsequently set at 0.3 and1.9% in a further pressure stage with a gas space (polymerization stage)as a function of the desired constant final degree of polymerization bytaking off the excess water. To this end the pressure in the gas spaceof this polymerization stage is regulated, preferably by regulating thewater content and/or the temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained by reference to working examples:

FIG. 1 shows diagrammatically the treatment of the cyclic dimers and thesubsequent implementation of the polymerization reaction.

In accordance with the sequence depicted in FIG. 2, undissolved dimersand oligomers present in recycled lactam are dissolved in the lactam.

EXAMPLES Working example 1

The working example describes a process in which recycled lactam istreated in accordance with FIG. 2 and then in accordance with FIG. 1.

For the preparation of polyamide 6 from recycled lactam, extractionwaters are concentrated to a mean solids content of 93% (long-termfluctuation range 91 to 95%). This recycled lactam (so-called returnlactam) consists of monomeric lactam, dimers, oligomers and water. Whenit is passed into the recycled lactam vessel 1, dimers and oligomers arenot sufficiently dissolved.

Using the heated pump 2, the recycled lactam is heated to 150° C. by wayof the heat exchanger 3. At the mixing temperature which is establishedin the recycled lactam vessel 1, all dimers and oligomers are dissolved.To establish a water content >3%, in this example between 5 and 9%, thepressure-limited regulating valve 5 is utilized. Any water whichevaporates is condensed in the condenser 6 and led off by way of thesubmersion unit 6a. Water is able to evaporate if the water content inthe crude lactam is >9%. By means of the metering pump 8, the recycledlactam is pressed via the heat transfer medium 9 into the dimertreatment stage 10. This and the following sequence is shown in FIG. 1.

In the heat exchanger 9, the recycled lactam is heated to at least 220°C. The dimer treatment stage 10 is a jacketed pressure apparatus whichis filled with product--without a gas phase--and is actively insulatedvia the jacket by means of heat transfer medium.

Under these conditions a polyamide melt is obtained, downstream of thedimer treatment stage 10, having a relative solution viscosity of from1.25 to 1.35 (solution viscosity based on polymer-sulfuric acid solutionwith 1 g of polyamide in 100 ml of 96% strength acid in comparison to96% strength sulfuric acid).

In order to permit the preparation of a polymer with constant andrelatively high viscosities, the physically dissolved water that ispresent in excess must be expelled from the melt in a polymerizationstage 13 down to a required water content which is dependent on thedesired final viscosity.

The valve 11 ensures that the pressure remains set at above the productboiling point in the dimer treatment stage 10. The product passes by wayof the evaporator 12 into the polymerization stage 13, which is operatedunder pressure at a water concentration of 1.7%. The evaporated waterleaves the system by way of the reflux column 14, from where the lactampresent in the ascending steam runs back into the polymerization stage13 again, and the valve 15 regulates the pressure of the process.

After the polymerization stage 13, the polyamide passes by way of valve16 and evaporator 17 into the polyamide post-condenser 18. In the bottompart of this post-condenser 18 the PA melt is cooled to 240° C.

Via the metering pump 20, the polyamide leaves the process with asolution viscosity of 2.5 and a water-extracted fraction of 9.9%.

Working example 2

In a polymerization unit for the preparation of polyamide 6, alactam-oligomer mixture is employed which originates fromdemonomerization stages and hence contains virtually no water.

In order to prevent the oligomers from precipitating, this mixture isheated by means of heat exchanger 3 to 150° C., as a result of which thedimers and oligomers dissolve. This solution can be transported withoutdifficulties over relatively long distances without the pipelinesbecoming blocked by precipitated oligomers.

The lactam-dimer-oligomer solution is heated, as in Working example 1,in the heat exchanger 9, but this time to a minimum of 240° C., and issubsequently introduced into the pressure stage of the invention, thedimer treatment stage 10, and treated. Between the heat exchanger 9 andthe dimer treatment stage 10, water is fed in by way of the valve 7 andthe melt is adjusted to a water content of 6% of physically dissolvedwater. The cyclic oligomers, especially dimers, are reactively cleavedin the pressure stage by hydrolysis.

The subsequent polymerization sequence takes place as described inWorking example 1. The resulting polymer has a solution viscosity of2.52 and a total extract of 9.9%.

Working example 3

In a polymerization plant, polyamide 6 is prepared from recycled lactamand fresh lactam.

To this end an approximately equal amount of fresh lactam is added tothe lactam-oligomer mixture recovered from demonomerization stages. Atthis mixing ratio, the oligomers dissolve in the lactam even at about120° C. Heating and subsequent treatment of the polymeric melt isperformed as described in Working example 1. The supply of water up to awater content of 6% is carried out as set out in Working example 2. Theresulting polymer has a solution viscosity of 2.61 and a total extractof 9.7%.

Working example 4

For the preparation of polyamide 6 from fresh lactam, fresh lactam isheated to 240° C. in a heat exchanger 9, as set out in Working example 1and in FIG. 1, and is then fed to the pressure stage of the invention,the dimer treatment stage 10. Between the heat exchanger 9 and dimertreatment stage 10 the water content of the fresh lactam is set at 3% byadding water. This effectively suppresses the formation of cyclicoligomers and dimers.

The subsequent polymerization sequence takes place as described inWorking example 1. The resulting polymer has a solution viscosity of2.70 and a total extract of 9.6%.

The table below compares the composition of the total extract of thepolymers prepared in accordance with the invention in Working examples1-4, with that of polymers prepared by customary techniques known fromthe prior art.

    __________________________________________________________________________                             Composition of the total extract in % based on                            Total                                                                             total extract                                                             extract                                                                           Monomer                                                                            Dimer                                                                              Trimer                                                                             Tetramer                              Reaction product                                                                          Process  (%) (%)  (%)  (%)  (%)                                   __________________________________________________________________________    Recycled lactam (Working                                                                  acc. to the invention                                                                  9.9 81.8 8.3  4.8  5.1                                   example 1 + 2)                                                                                      11.2. to the prior art                                                                                  6.7                           50% recycled lactam                                                                                acc. to the invention                                                          9.7                                                                                                     4.9                           50% fresh lactam                                                                                    10.5. to the prior art                                                                                  6.5                           (Working example 3)                                                           Fresh lactam                                                                                        9.6   acc. to the invention                                                                             4.2                           (Working example 4)                                                                               acc. to the prior art                                                           10.0                                                                                                    6.5                           __________________________________________________________________________

We claim:
 1. A process for continuously preparing polyamide 6 with a lowdimer content from ε-caprolactam (fresh lactam) and/or from recycledlactams (so-called return lactams) from extraction stages and meltdemonomerization stages, in a plurality of process stages, whichcomprises supplying recycled lactam to a first and subsequently to asecond pressure stage, fresh lactam to the first and subsequently to thesecond pressure stage or only to the second pressure stage, treatmenttaking place in the first pressure stage (dimer treatment stage) byhydrolysis in the liquid melt phase at water contents of from 3 to 15%and at temperatures in the range from 220° C. to 280° C., andsubsequently supplying the polyamide melt to the subsequent furtherpolymerization process in the second pressure stage, in a pressure stagewith a gas space (polymerization stage), while establishing a lowerconstant water content.
 2. The process as claimed in claim 1, whereinrecycled lactam and/or fresh lactam are introduced into a closed system.3. The process as claimed in claim 1, wherein a melt of fresh lactamand/or of recycled lactam is supplied in the presence of water to thefirst pressure stage (dimer treatment stage) before passing throughreaction stages of a polymerization unit.
 4. The process as claimed inclaim 1, wherein the water content in the dimer treatment stage whenonly fresh lactam is employed is set in the range from 3-10%.
 5. Theprocess as claimed in claim 1, wherein in the melt of fresh lactam andrecycled lactam or recycled lactam alone the oligomers are dissolved inthe lactam before the polymerization mixture enters the first pressurestage, and the solution temperature of the oligomers in the lactam isset, depending on the concentration of the dimers in the solution, atbetween 120° C. and 180° C.
 6. The process as claimed in claim 1,wherein the water content for the hydrolysis of the cyclic oligomers inthe melt of fresh lactam and recycled lactam or recycled lactam alone isheld in the first pressure stage, corresponding to the existing watercontent of the recycled lactam, at from 6 to 15%.
 7. The process asclaimed in claim 1, wherein the water content in the polymeric melt isset at 0.3-1.9% in the further pressure stage with a gas space(polymerization stage) as a function of the desired final degree ofpolymerization by taking off the excess water.
 8. The process as claimedin claim 4, wherein the water content in the dimer treatment stage whenfresh lactam is employed is set in the range from 3.1-4.5%.